1. Field of the Invention
The present invention relates to a method of measuring a groove formed by a dicing apparatus, by an ultrasonic detector, and a dicing apparatus provided with an ultrasonic detector and capable of measuring the depth of a groove and, more particularly, to a method capable of very accurately measuring the depth of a dicing groove cut in a workpiece in a full-cut mode in which the dicing groove is cut as deep as the periphery of a cutting blade which reaches the surface of an adhesive sheet fixedly holding the workpiece to dice the workpiece completely and the depth of a dicing groove cut in the workpiece in a half-cut mode in which the dicing groove is cut so that the workpiece is not diced completely, and to a dicing apparatus for carrying out the same.
2. Description of the Related Art
A dicing apparatus, which cuts a narrow groove in a workpiece with a cutting blade rotating at a high rotating speed, is applied widely to cutting of semiconductor wafers provided with semiconductor devices thereon, into semiconductor chips. A dicing apparatus as applied, by way of example, to cutting a groove in a semiconductor wafer will be described.
In the dicing apparatus, a stage supporting the semiconductor wafer is moved relative to a cutting blade rotating at a high speed to cut a groove in the semiconductor wafer. Although the semiconductor wafer is fixedly supported on the stage, generally, the semiconductor wafer is stuck on an adhesive sheet, and then the adhesive sheet adhesively holding the semiconductor wafer is held fixedly on the stage to prevent semiconductor chips formed by dicing the semiconductor wafer from scattering during processing. A workpiece other than a semiconductor wafer may be directly attached to the stage with an adhesive. An adhesive sheet is stuck to one surface of a frame having a central hole formed by punching and having a diameter greater than that of the semiconductor wafer, and the semiconductor wafer 1 is stuck on the adhesive surface of the adhesive sheet, and the frame thus holding the semiconductor wafer is fixedly mounted on the stage. In production processes of semiconductor devices, a semiconductor wafer is transferred from one process to the next in a state described above.
When cutting a groove in the semiconductor wafer, the groove is cut, depending on the requirements of the following processes, in a full-cut mode in which the groove is cut so that the semiconductor chips are diced completely or in a half-cut mode in which the groove is cut in the semiconductor wafer so that the depth of the groove is smaller than the thickness of the semiconductor wafer. In the full-cut mode, the position of the cutting blade relative to the stage is adjusted before starting the cutting operation so that the periphery of the cutting blade will reach the adhesive sheet. In the half-cut mode the position of the cutting blade relative to the stage is adjusted before starting the cutting operation so that the periphery of the cutting blade will not reach the bottom surface of the semiconductor wafer.
The accurate control of the depth of the groove is essential to the dicing process to avoid producing defective products. When adjusting the position of the periphery of the cutting blade, the periphery of the rotating cutting blade is brought into contact with the surface of the stage or a surface at a fixed height from the surface of the stage, the position of the periphery of the cutting blade is measured, and the position of the cutting blade is adjusted taking into consideration the thickness of the adhesive sheet so that the position of the periphery of the cutting blade and the surface of the stage satisfy a predetermined relation. Since the cutting blade is abraded with the progress of the dicing process, the position of the periphery of the cutting blade is measured and corrected whenever necessary for the further accurate control of the groove.
However, since the dicing operation must be interrupted for the measurement of the position of the periphery of the cutting blade, the frequent measurement of the position of the periphery of the cutting blade will deteriorate the efficiency of the cutting process.
Therefore, the position of the periphery of the cutting blade is corrected on the basis of the empirically predetermined relation between the quantity of cut material and the abrasion of the blade. When the position of the periphery of the cutting blade is thus corrected, the position of the periphery of the cutting blade can be more correctly controlled. However, since the abrasion of the blade is dependent on its quality and the cutting conditions are variable, such a method of controlling the position of the periphery of the cutting blade is not completely satisfactory.
The applicant of the present patent application proposed a dicing apparatus capable of measuring the shape of the groove, i.e., the depth and width of the groove, with an ultrasonic detector in Japanese Unexamined Patent Publication No. 4-267106. In this dicing apparatus, the ultrasonic detector emits ultrasonic waves intermittently, detects the reflected ultrasonic waves, and calculates the distance between the ultrasonic detector and a surface facing the ultrasonic detector on the basis of the time interval between the emission of the ultrasonic wave and the return of its echo from the surface, the phase difference between the emitted ultrasonic waves and their echo, or both the time interval and the phase difference. The shape of the groove and, naturally, the depth of the groove can be measured by moving the ultrasonic detector so as to scan the groove formed in the wafer. In the dicing apparatus, pure water is supplied as a cutting fluid. The pure water filling the space between the ultrasonic detector and the object surface improves the propagation efficiency of the ultrasonic waves and measuring accuracy. This dicing apparatus is capable of detecting chipped parts, such as a chipped part in the edge of the groove.
Since the ultrasonic detector is capable of measuring the shape of the groove even if pure water is used as a cutting fluid, the cutting process need not be interrupted for the measurement and the possibility of continuous monitoring of the shape of the groove reduces faulty cutting.
Since the ultrasonic detector emits the ultrasonic wave intermittently and detects the reflected ultrasonic waves, the detection efficiency is greatly dependent on the ultrasonic reflectance of the surface facing the ultrasonic detector. Since the ultrasonic reflectance of the adhesive sheet is far smaller than that of the wafer 1, the reflection of the ultrasonic waves by the groove is reduced greatly when the groove is cut through the wafer into the adhesive sheet in the full-cut mode and hence the accurate measurement of the groove is difficult in this state. Such a difficulty may be overcome by enhancing the detection sensitivity, however, since the ultrasonic waves emitted by the ultrasonic detector spread to some extent and the reflectance of the side surfaces of the groove 100 is comparatively high, the accurate measurement of the depth of the groove is difficult because the ultrasonic waves reflected by the side surfaces of the groove act as noise.